An energy storage system is a system which separately stores residual electric power and supplies the stored electric power in a timely manner. The energy storage system may be largely divided into a physical energy storage system and a chemical energy storage system depending on the manner of storage of energy. Examples of the representative physical energy storage system include a pumping-up power generation system, a compressed air storage system, a flywheel and the like. Examples of the chemical energy storage system include a lithium battery, a lead storage battery, a NaS battery and the like.
The energy storage system has been extensively studied in that the power supply/demand problem can be solved by storing electric power wasted at night and using the stored electric power in a peak time zone.
As one example of a small-size energy storage system, there is known a battery for an electric vehicle. The electric vehicle is a motor vehicle driven by an electric motor and is mounted with a large-capacity battery. In the past, a lead storage battery has been used as the battery for an electric vehicle. Nowadays, a nickel hydrogen battery and a lithium battery are mainly used. It is expected that the lithium battery will be mainly used in the future.
The lead storage battery used in the past has an advantage in that the price thereof is relatively low and the reliability thereof is high. However, the lead storage battery is problematic in that the output per unit weight is low, the volume is large, the output voltage is reduced when used for a long period of time, the discharge rate is low, and the lifespan is shortened due to overheating when frequently exposed to a load requiring a high output. Thus, the lead storage battery is not preferentially selected in an electric vehicle and the use thereof is evaded. In addition, the lead storage battery has a problem in that it is not suitable for charging the electric energy recovered through regenerative braking.
The lithium battery is a battery which is higher in output and density than other batteries. Thus, the lithium battery is spotlighted. However, the lithium battery is very expensive. The performance of the lithium battery depends largely on the temperature. In particular, electrolyte decomposition occurs at a high temperature. As a result, the lifespan of the lithium battery is sharply reduced. Furthermore, the lithium battery has a risk of fire and explosion. In order to improve such problems, methods for cooling a battery are disclosed in Korean Patent Application Publication Nos. 2010-0001877, 2003-0100891 and 2003-0100893.
The lead storage battery used at the present time is capable of storing electric energy of about 1 kWh per 10 kg. An electric vehicle can run 5 to 10 km using electric energy of about 1 kWh. Thus, in order for an electric vehicle to run a mileage of about 700 km, a lead storage battery of about 1 ton is necessary even if the lead storage battery has a high density. For that reason, a secondary battery of low density such as a lead storage battery or the like cannot be used as a battery for an electric vehicle.
However, an electric vehicle capable of driving about 100 km when charged once has a short mileage. Therefore, such an electric vehicle does not have to use a high-density battery. The use of a low-priced lead storage battery provides an advantage in that the cost is saved and the risk of fire and explosion is eliminated, which makes it unnecessary to use a complex cooling structure. Moreover, it is not necessary to consider the risk of fire and explosion when disposing the lead storage battery. This provides an advantage in that the lead storage battery can be disposed with a higher degree of freedom.
However, as described above, if the lead storage battery is used for a long period of time, the output voltage thereof becomes low. Therefore, the electric vehicle is difficult to run. Since the lead storage battery is lower in output than the lithium battery, the lead storage battery is difficult to cope with a case where a high output is required, such as a case where an electric vehicle restarts after stoppage or a case where an electric vehicle runs on a sloping road. The lead storage battery has a problem in that the lifespan thereof is shortened when the lead storage battery is frequently exposed to a load requiring a high discharge rate. In addition, the lead storage battery has a problem in that it cannot be used for charging electric energy generated by regenerative braking.
In the case of a conventional hybrid battery system that alternately uses different types of batteries, the amount of energy is sharply changed according to the change in the type of the battery used. This poses a problem in that a vehicle occupant or a user may feel a shock generated due to the change in the amount of energy. Moreover, there is a problem in that the energy efficiency is low.
Furthermore, in the case of using a hybrid battery system in a plug-in hybrid car, if driving and charging are simultaneously performed while driving a range extender, a problem is posed in that a motor vehicle has to be driven by a lead storage battery while charging a lithium battery.